In recent years, the demand for secondary batteries has increased in various applications as power sources for PCs, video cameras, cellular phones, and the like, or as power sources for electric vehicles and power storage media. Among secondary batteries, particularly lithium-based secondary batteries have higher capacity density than other secondary batteries and can operate at higher voltages, and thus are used for information-related devices or communication devices as secondary batteries for miniaturization and light weight. Recently, the development of high power and high capacity lithium-based secondary batteries for electric vehicles or hybrid vehicles is underway.
A typical lithium-based secondary battery consists of a positive electrode (cathode), a negative electrode (anode) and an electrolyte containing lithium salt interposed therebetween, and this electrolyte may be a non-aqueous liquid electrolyte or a solid electrolyte. When the non-aqueous liquid electrolyte is used as an electrolyte, since the electrolyte penetrates into the positive electrode, it is easy to form an interface between the positive electrode active material constituting the positive electrode and the electrolyte, and accordingly, the electrical performance is high.
However, since the lithium-based secondary battery uses a flammable organic solvent in the liquid electrolyte, in order to prevent ignition or rupture that may occur due to over current caused by short, it may be necessary to install a safety device. In addition, in order to prevent such a phenomenon, there are cases where the selection of the battery material or the design of the battery structure is restricted.
Therefore, the development of an all-solid battery using a solid electrolyte instead of a liquid electrolyte has been underway. Since the all-solid battery does not contain a flammable organic solvent, the all-solid battery has the advantage of simplifying the safety device and thus is recognized as a superior battery in terms of manufacturing cost or productivity. In addition, since it is easy to laminate, in series, a pair of electrode layers including a positive electrode (cathode) layer and a negative electrode (anode) layer, and a junction structure including a solid electrolyte layer lying between these electrode layers, the all-solid battery is expected to be a technology capable of producing a high-capacity and high-output battery with stability.
On the other hand, it is known that, in the all-solid battery, the contact resistance between the particles of the active material particles responsible for the cell reaction or between the active material particles and the solid electrolyte particles greatly affects the internal resistance of the battery. Particularly, as the volume change of the active material due to repetition of charging and discharging occurs, the contactability between the active material and the solid electrolyte or the conductive material is reduced, and there is a tendency that an increase in internal resistance or a decrease in capacity or the like occurs easily. Accordingly, various techniques which improve contactability between particles of the active material or the solid electrolyte, and suppress an increase in internal resistance and the like have been proposed.
For example, there is an attempt to improve the performance of the all-solid battery by paying attention to the interface between the positive electrode active material and the solid electrolyte material. For example, in a research paper by Narumi Ohta et al. (“LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries”, Electrochemistry Communications 9 (2007), 1486-1490), LiNbO3 (lithium niobate) is described as a material to be coated on the surface of LiCoO2 (positive electrode active material). This technique aims to obtain a high-power battery in such a manner that the surface of the LiCoO2 is coated with LiNbO3 to suppress the reaction between LiCoO2 and the solid electrolyte material, thereby reducing the interfacial resistance between LiCoO2 and the solid electrolyte material.
In addition, when the negative electrode active material reacts with the solid electrolyte material, a high resistance portion is generally formed on the surface of the negative electrode active material, thereby increasing the interfacial resistance between the negative electrode active material and the solid electrolyte material. In order to resolve this problem, Japanese Laid-Open Patent Publication No. 2004-206942 discloses an all-solid battery wherein a second solid electrolyte layer, which does not chemically react with the first solid electrolyte and has lower ion conductivity than the first solid electrolyte layer (sulfide based solid electrolyte material), is formed between the first solid electrolyte layer and a negative electrode made of metal lithium. This application attempts to inhibit the reaction between the first solid electrolyte layer and the metal lithium through formation of the second solid electrolyte layer with low ion conductivity.
However, attempts to improve the performance of the all-solid battery have failed to adequately lower the interfacial resistance between the positive electrode or negative electrode active material and the solid electrolyte material, thereby making them unsuitable for manufacturing high power and high capacity batteries.